Beta particle penetrating ability. At left is a disc of strontium 90 (green), a radioactive isotope of the element, and at right is a Geiger counter. Between the two is a piece of lead. The strontium 90 disc is emitting beta particles, which are electrons emitted by unstable atomic nuclei. Very few beta particles are able to penetrate the lead and reach the detector in the Geiger counter. To see this apparatus without the lead shield see clip K004/4567.